One that goes before and indicates the coming of another.
Scientists are trying to identify special geological activity that may be a precursor to an earthquake, which will help them predict the quake's size, time, and location.
A precursor is literally a "forerunner," but the two words function a little differently. A forerunner may simply come before another thing, but a precursor generally paves the way for something. The Office of Strategic Services in World War II was the precursor of today's Central Intelligence Agency. The blues music of the 1930s and 1940s was a precursor to the rock and roll of today. The war in Bosnia could be a precursor to more armed conflict in Eastern Europe and the former Soviet Union.